Prescription ADHD medications like Adderall, Ritalin, and Vyvanse are becoming increasingly popular for overworked and overscheduled college students — who haven’t been diagnosed with ADHD.

“Our biggest concern … is the increase we have observed in this behavior over the past decade,” says Sean McCabe, research associate professor at the University of Michigan Substance Abuse Research Center.

Full-time college students were twice as likely to have used Adderall non-medically as their counterparts who were not full-time students, according to a National Survey on Drug Use and Health report released in 2009.

“When we look at upperclassmen, the number really begins to jump,” says Alan DeSantis, professor of communications at the University of Kentucky who has conducted research on stimulant use in college. “The more time you stay on campus, the more likely you are to use.”

ADHD stimulants “strengthen the brain’s brakes, its inhibitory capacities, so it can control its power more effectively,” said Dr. Edward Hallowell, a psychiatrist and ADHD expert. “They do this by increasing the amount of certain neurotransmitters, like dopamine, epinephrine, and norepinephrine.”